Attaching a peripheral device to a telephone via the handset (or headset) port creates a number of difficulties. The most obvious difficulty is that the handset port was only designed with a very limit purposes i.e. receiving and sending voice respectively to a speaker and from a microphone. Thus, there is little or no control signaling through handset ports. This is not a problem if only handset sets were connected to the handset ports. However, this is increasingly not the case.
The most obvious example is that of a headset and a handset connected to a handset port of a telephone set. Thus, the question arises, how is the telephone set to determine which device, the headset or the handset, is active. Typically, the handset must be left off hook either by some mechanical device or by a secondary cradle. Gancarcik in U.S. Pat. No. 5,832,075 disclose a further solution by monitoring the telephone bias voltage of the handset ports. Thus, the telephone set takes action when a switch on the headset interrupts the bias.
The solution disclosed by Gancarcik works well with a passive device such as a headset. However, if one wishes to use more sophisticated peripheral devices such as an add-on speakerphone or a wireless headset, it is desirable for the telephone set to have bi-directional control signaling or communication with the peripheral devices. It is therefore desirable to provide a signaling system, which addresses, in part, some of the shortcomings of handset ports noted above.